... started a question at Stackexchange and asked a ‚well-established‘ lawyer off the record. Both answers confirmed my resolution to choose the AGPLv3.

The trick is in my words: there was never a software under GPLv3 until now, AsciiDoc is GPLv2 or later. There is no ‚switching from GPLv3 to AGPLv3‘, because there is/was no GPLv3! So, according to the ‚compatibility matrix‘, you may upgrade from GPLv2 to AGPLv3. All right.